Ed,

I do not know if there is a right or wrong way as long as what you do works and is comfortable.

I replaced the stiff harness leather on mine with a softer leather, cut it to an angle and loop size that fits my thumb.

I hold the accordion with my thumb print against the finger board. I hold it on my right knee and move only the left side. This works for me but others may find another way more comfortable.

Good luck
RPr

Thanks for your comments.
They are very helpful.

.... Ed